The present invention relates to a system for controlling a fluid pressure, and more specifically to a hydraulic modulator which normally allows a fluid pressure to flow from a pressure supplying means to a hydraulic actuator, and which interrupts the fluid communication between the pressure supplying means and the hydraulic actuator and controls the fluid pressure supplied to the hydraulic modulator independent of the pressure of the supplying means when the fluid pressure of the supplying means becomes unable to operate the hydraulic actuator property.
Japanese Patent (post examination) Publication No. 56-254 shows one conventional hydraulic modulator for an anti-skid brake control system. This modulator uses a ball valve for disconnecting a wheel cylinder and a master cylinder, and a pneumatic diaphragm mechanism for operating the ball valve and modulating the fluid pressure of the wheel cylinder. However, the diaphragm mechanism of this modulator must be large enough to produce a required mechanical force.